To investigate the physico-chemical basis of the carious process in relation to the movement of ions, such as calcium and phosphate, across the enamel surface. Studies will concentrate on the demonstrated caries inhibitory effect of fluoride, and to a lesser extent strontium, on caries reduction and/or remineralization. Methods of investigation will be by solubility of synthetic and natural tooth enamel apatites with and without ions such as fluoride, strontium, magnesium and barium. Artificial lesions produced in natural and synthetic apatites will be studied for movement of ions across the enamel surface by diffusion and secondly for the mineralization potential of fluoride and strontium. X-ray diffraction, electron microscopy, and microhardness tests, among others, will be used to study the movement of ions across the enamel lesion during solubility and/or remineralization studies.